Enhanced electron conduction at grain boundaries in aged CdTe photovoltaics as observed by conductive tomographic AFM

Artificial aging of photovoltaics are the most cost-efficient method of ensuring product warranties. Yet, the influence artificial aging on electrical properties within the microstructure of Cadmium Telluride (CdTe) and other polycrystalline photovoltaics remains largely unknown. Partly because characterization techniques have been unable to image electrical pathways through three-dimensional grains and grain boundaries with nanoscale resolution. Accordingly, this work employs a novel conductive and tomographic variation of atomic force microscopy. CdTe thin-film solar cells are progressively mechanically ablated through the full thickness, while photocurrents generated with in-situ illumination and biasing are simultaneously mapped. Devices that have been exposed to accelerated aging show two differences with pristine samples. First, we observe enhanced electrical conduction along grain boundaries, consistent with a loss of open circuit voltage. Second, we observe islands of reduced photocurrent, located along hole-conducting planar stacking faults, consistent with a loss of short circuit current. These defects are likely a result of recombination, or the defects result in recombination, thusly reducing photocurrent.